add magit-log-edit-nop
magit-key-mode.el: improve doc-strings

Also use nil for empty list, not '().
magit-key-mode-build-exec-point-alist: move definition
magit-key-mode-redraw: fix indentation, line length
magit-key-mode.el: add section headings
magit-key-mode.el: move defvars into appropriate sections

This might be somewhat controversial, but it (along with adding the
section headings in an earlier commit) will help me to reason about
the existing code, while working on a new implementation.  Also
defvars weren't consistently at the beginning of the Code section
anyway.
rename magit-key-mode-{ key-maps => keymaps }

For consistency with how this is written everywhere else:)
rename magit-{ log => pre-key }-mode-window-conf